Interaction-enhanced antiferromagnetic correlations in a Kondo ladder with a gapped band structure

We investigate magnetic correlations in the two-leg Kondo-Hubbard ladder using the density-matrix renormalization group method. We focus on a parameter regime in which the noninteracting electronic system has a narrow indirect band gap. In this regime, there is no Fermi surface, and the conventional Fermi-surface-based Ruderman-Kittel-Kasuya-Yosida picture is not directly applicable; magnetic correlations between localized spins become weaker as the noninteracting band gap increases. We find that a moderate on-site Coulomb interaction enhances the antiferromagnetic correlations between localized spins. This enhancement, which is also seen in the staggered magnetization induced by a pinning field at the edges, occurs for both ferromagnetic and antiferromagnetic Kondo couplings. Our results show that electron-electron interactions can enhance magnetic correlations between localized spins even when the underlying noninteracting electronic structure is gapped, suggesting an interaction-driven route to magnetic instabilities in narrow-gap Kondo-lattice systems.